Myth of the Week Series
This will be one of my shorter posts, but I found this topic to be quite interesting!
Question: Why Do a Bird’s Knees Bend Backwards?
Those of us who live near the water have often observed a scene like this:
A juvenile little blue heron1 walking along my seawall on South Siesta Key. Note the backward bend of the legs. (Nikon D5600; FL = 165mm; 1/1600 sec; f/8; ISO 320; EC = 0)
In the above photo, we have a garden-variety seabird strolling along the water, very likely looking for a tasty afternoon snack. Note the curious direction in which the legs bend: opposite to that of our own knees. You can check this out for any number of other birds: flamingos, herons, and even robins. But why is this so? Why would their knees bend backwards?
As they say in the lead-in to TV news stories: “The answer may surprise you!” In this case, the answer probably will.
Answer: Those Aren’t the Knees, They’re the Ankles!
Actually, a bird’s knees do bend the same way as ours. Perhaps an anatomical sketch would help:
In the above illustration, the bird is facing to the left, as can be verified, for example, by noting the pattern of the “toes” on the ground. (Adapted from an illustration by Darekk2 – Own work, CC BY-SA 3.0, Link)
The knee (red call-out) bends forward, the same as ours. The confusion arises from the fact that a bird’s knee is usually hidden beneath the wing feathers. The ankle, however, (blue call-out) is right out there for all to see.
This can be compared with the analogous parts of the human anatomy:
A sprinter launching herself out of the starting block. Note the forward bending knee and the backward-bending ankle. Note that the “heel” is simply a bone that protrudes from the back of the ankle joint. (Photo adapted from tableatny – BXP135624, CC BY 2.0, Link)
In the photo, we show a sprinter leaping into action at the start of a race. Compare this with the illustration above it. If you examine the call-outs of the same color, you will notice that the basic structure is the same for a bird and a human: the knee bends forward and the ankle bends backwards. It’s a simple matter of locomotion.
If you are surprised by all this, take heart. Even an esteemed, well-respected magazine like Scientific American got it wrong!
Why the Design Difference?
As you’ve probably guessed, the real difference is in the relative lengths of the various bones in the leg. This varies greatly from tetrapod to tetrapod, and even within a given biological class, such as Aves (i.e., birds). The link cited just above has some nice illustrations of this. But why the difference?
Truth in Blogging Statement:
Evolutionary biology is not my strong suit. (I’m a laser physicist.) An expert in the area of tetrapod anatomy can probably wax eloquent on the subject for many pages. In fact, good discussions of plantigrade versus digitigrade animals can be found on Wikipedia. (Hint: Birds, cats, and dogs walk on their toes; humans walk with heel and ankle flat on the ground.)
That said, I have to believe that part of the answer follows from the different ways that animals obtain their food. (Snagging a meal is very important!) In the old days, humans got their food by hunting other mammals (usually done by the males), and gathering fruit and nuts from trees and bushes while carrying babies (usually done by the females). This is best achieved with a mechanically stable, weight-bearing posture. (Plantigrade feet have a large surface area on the ground.)
Many birds, on the other hand, grab their food low to the ground (or in the water). Robins, for example, are natural ground feeders. In this case, it is a huge advantage to be able to bend forward and down easily. The same is true for wading birds:
A snowy egret has just bagged a dinner of fresh sushi! I took this photo in the shallows of Little Sarasota Bay. (Nikon D40; FL = 185mm; 1/125 sec; f/9; ISO 400; EC = +0.3)
If you look carefully at the photo above, you can see the egret’s bent legs under the surface of the water. The egret nabs the fish by bending low over the water and lunging downwards.
Any biologists out there? Feel free to add your comments (or correct my mistakes!) in the Leave a Reply section below. ♦♦♦
1 No, there’s nothing wrong with your eyes. The juvenile little blue heron is all-white, except for “vague dusky tips to the outer primaries”, and a two-toned bill.